In recent years in which an inkjet printer (an image forming apparatus) which has installed an inkjet head (a liquid droplet ejecting head) has been highly rated for high picture quality, low price, responsiveness to high speed (able to deal with fast printers as well as slow but inexpensive printers by increasing and decreasing the number of nozzles) and popularized, a further improvement in picture quality and reliability and a further reduction in cost and size have been demanded.
An inkjet head, which includes a nozzle hole which ejects ink liquid droplets; an ejecting liquid chamber (also called a pressurizing liquid chamber, a pressure chamber, an ink flow channel, etc.) which is communicatively connected to the nozzle hole; and a pressure generating unit which generates pressure which pressurizes ink within the ejecting liquid chamber, pressurizes ink within the ejecting liquid chamber with the pressure generated in the pressure generating unit to eject the ink liquid droplets from the nozzle hole.
A pressure generating unit is known which ejects ink droplets by using an electromechanical converting element such as a piezoelectric element, etc., to deform a vibrating plate which forms a wall face of the ejecting liquid chamber.
Ink used for the inkjet head generally has a property that the viscosity changes due to a temperature change, so that an amount of ink ejected changes with temperature when ejecting pressure is constant regardless of a change in the ambient temperature. Thus, there is a problem that a recording dot diameter on a recording medium changes with temperature, and, when the ambient temperature deviates from a predetermined temperature range, good print results are not obtained, causing picture quality to degrade.
As a configuration which changes ejecting pressure depending on a change in the ambient temperature, Patent document 1 discloses a configuration which uses a material (a temperature measuring resistive body) whose resistance value changes with the ambient temperature in a part of a wire which transmits a signal for driving a piezoelectric element. For example, when the ambient temperature rises, the viscosity of ink decreases, so that ejecting pressure required for ejecting a certain amount of ink decreases. In a configuration disclosed in Patent document 1, when the ambient temperature rises, the resistance value of the temperature measuring resistive body increases, increasing a voltage drop in the wire, thereby decreasing a voltage which drives a piezoelectric element as a result. In this way, an amount of deforming of a piezoelectric element to which a voltage is applied decreases and an ejecting pressure also decreases. In other words, in the configuration in Patent document 1, for a higher temperature environment in which the viscosity of ink decreases and required ejecting pressure decreases the ejecting pressure may be made lower and for a lower temperature environment in which the viscosity of ink increases and required ejecting pressure increases the ejecting pressure may be made higher.
However, with the configuration disclosed in Patent document 1, an amount of change in ejecting pressure required due to a change in the viscosity of ink with a change in temperature and an amount of change in ejecting pressure of a piezoelectric element due to a resistance value of the temperature-measuring resistive body with a change in temperature do not necessarily match. Then, it is very difficult to cause the amount of change in the ejecting pressure required with the change in temperature and the amount of change in the ejecting pressure of the piezoelectric element to match in all ambient temperature areas.
As different configurations which change the ejecting pressure depending on a change in the ambient temperature, Patent documents 2 and 3 disclose a configuration including a temperature detecting unit in which a voltage is applied to a temperature measuring resistive body (a thermistor) and a change in a resistance value therein is measured to detect a temperature at a location at which the temperature measuring resistive body is arranged; and a pressure control unit which controls an output of a pressure generating unit such as a piezoelectric element, etc., based on detected results of the temperature detecting unit.
In such configurations, information on the resistance value of the temperature measuring resistive body and on an output value of the pressure generating unit that is suitable at a temperature corresponding to the resistance value is input in advance into a data table included by the pressure control unit. Then, an output of the pressure generating unit is controlled based on information in the data table.
Such configurations make it possible for the pressure generating unit to output ejecting pressure required in the respective temperatures when ejecting pressure required changes due to a temperature change.